Switching power supplies include a power switch for alternately coupling and decoupling an unregulated input voltage source to an output terminal to which a load is coupled. An output filter, including an inductor and a capacitor, is coupled between the power switch and the regulator output terminal and averages pulses of the input voltage to provide a DC output voltage at the output terminal. A feedback circuit controls the duty cycle of the power switch in order to maintain the output voltage at a substantially constant level.
As is known in the art of switching power supplies, load variations can detrimentally affect the regulation of the DC output voltage. That is, load increases tend to cause the output voltage to drop whereas load decreases tend to cause the output voltage to rise. When load variations occur relatively slowly, the regulator may respond quickly enough to prevent the output voltage from deviating from an acceptable tolerance range. However, in instances where load variations occur quicker than the rate of response of the regulator, the output voltage may disadvantageously deviate from the acceptable range.
One factor which limits the speed with which the regulator responds to load variations is the size of the output filter inductor. That is, the rate of change of current through the inductor is inversely related to the inductance. Thus, one way to improve the regulator response time is to decrease the size of the output inductor. However, as the inductance is decreased, the frequency at which the power switch is operated must generally increase, thereby increasing switching losses.
Another way to improve the regulator response time is to increase the size of the output capacitor. That is, the larger the output capacitor, the more charge stored and available for accommodating varying load requirements. However, cost and size considerations, as well as increased losses related to the equivalent series resistance associated with capacitors, limit the feasibility of increasing the size of the output capacitor.